Peter Ramm wrote:

Is 4sqmm being phased out

No - but it is perhaps less commonly required than other sizes, so might
not be stocked by outlets that only want to handle fast-moving
high-volume lines.

- surely some of the connectors (eg cooker) which would previously
take 2 * 4sqmm cables will not take not 2 * 6sqmm?

The terminal capacity of the MK K5045 cooker connection unit is given as
2 x 10 mm^2 (including the input wiring), so 3 x 6 mm^2 ought to fit.
In any case these units aren't really intended to feed more than one
output cable. If there are two appliances to connect, use two
connection units side-by-side. Of course that doesn't help with overall
terminal capacity if you loop the input from one to the other, but it
will if you feed them separately from the control switch (provided that
has sufficient capacity...).

--
Andy